Tripper,
Usually the larger bottles like the 32 and 40-ounce size have larger, thicker bottoms. If you can find an old abandoned house site you can also find discarded cider and moonshine bottles with large, thick, flat bottoms. I would search some local recycling centers and search thru the bottle dumpsters. I've found some nice bottles there and made some cool points out of them. Just be careful cause there's usually a lot of broken shards and you can get bad cuts if ya aren't careful.